Ink jet printing has been currently identified as one of the most successful candidates for the technology of choice in the digitally controlled, electronic printing market. Two prominent forms of this technology are drop-on-demand (DOD) and continuous ink jet (CIJ). CIJ technology was identified as early as 1929, in U.S. Pat. No. 1,941,001 issued to Hansell. In the 1960s, CIJ printing mechanisms were developed that made use of acoustically driven print heads to break off ink droplets that would be appropriately deflected by electrostatics. Since this time, there have been numerous advances in the implementation of CIJ printers, including the use of CMOS/MEMS integrated print heads with resistive heating elements to break up a fluid column into drops. The drops created by heat pulses may be positioned through the use of techniques such as air deflection. These concepts have been disclosed in U.S. Pat. Nos. 6,079,821, 6,450,619, 6,863,385.
Using heat to break up the drops allows a greater degree of freedom in controlling individual streams of fluid, as opposed to the use of acoustic control to break up drops uniformly at all nozzles of the print head. Furthermore, the use of air deflection in place of electrostatics reduces the requirements placed on ink properties, for example conductivity requirements. By adjusting the electrical potentials applied to the resistive heater with respect to time, one can control the size of the drops that are produced. Heat may be applied to the fluid, via an adequate electrical potential supplied to the print head heaters, frequently to create small drops. Less frequent application of heat pulses generates larger drops, as described in U.S. Pat. No. 6,575,566. Therefore, specific electrical waveforms may be created to apply to the heaters of the print head as necessary.
The application of the heat pulses, however, has undesired effects under certain conditions. These effects are evident when dealing with larger sized drops, for example, a drop formed by two heat pulses widely spaced in time. Fluid instabilities appear within regions of the large drop that are meant to be contiguous and cause the drop to break up, as can be appreciated by an expert in fluid dynamics. The break-up of large drops is generally deleterious to high quality printing, since the drop volumes are not well controlled and thus the drops may not be used as intended. When the large drops break up into smaller pieces, they generally travel an additional distance in space before they re-form by joining, as is also known in the art of fluid dynamics. The total distance the stream must travel from the printhead surface in order to form controlled drops that can be used as intended in printing is termed the “coalescence length.” Generally, it is desired that the coalescence length be minimized. For example, in the printing methods using air deflection to position drops (U.S. Pat. Nos. 6,079,821, 6,450,619, 6,863,385) the accuracy of positioning degrades if the large drops break up into smaller drops, or if the coalescence length is too long. This is because drops deflect differently in the air depending on their size, as can be appreciated by one knowledgeable in classical mechanics; and because a long coalescence length requires the receiver to be remote from the printhead, further degrading drop placement accuracy, as is well known in the art of inkjet printing. Clearly there is a need in the industry of inkjet printing to provide well-controlled drops and to minimize the distance of the receiver to the printhead.